The present invention relates to a method for welding thermoplastic resin, and more particularly to a method for welding thermoplastic resin with a high frequency welder, which enables the welding of thermoplastic resin not capable of being welded by a conventional method for welding with a high frequency welder.
As conventional methods for welding thermoplastic resin having a low dielectric loss which cannot be welded with a high frequency welder, generally, there have hitherto been known a method for welding with an impulse sealer, a method for welding with a heated metallic mold and a method for welding with an ultrasonic generator.
The method for welding with an impulse sealer is a method, wherein electric current is applied to a metallic heating resistor having a flat surface to heat the resistor and sheets of thermoplastic resin causing welding by the generated heat. The above method has an advantage in that the temperature of the heating resistor can be easily adjusted. However, the above method has a disadvantage in that, for example, when a tube having low hardness is inserted between two sheets to be adhered and the tube and the sheets are welded together, the tube is sometimes deformed and the interface between the tube and the two sheets is not completely sealed and thereby a leak is generated.
The method for welding with a heated metallic mold is a method, wherein the metallic mold is heated and adherends of thermoplastic resin are welded by the generated heat to each other. The above method has an advantage that adherends having various kinds of shape can be welded to each other by processing the metallic mold so as to fit with the shape of the adherends. However, since the temperature of the heated metallic mold is influenced by the temperature of the surrounding and is sometimes lowered with repeating the above method for welding, it is difficult to control the temperature of the metallic mold. For example, in case of the temperature of the metallic mold being excessively high, the adherends are molten and deformed and show a defective appearance. Also, when the temperature of the metallic mold is low, it takes a long period of time for welding the adherends and as a result, productivity is deteriorated or sometimes the adherends are not completely and uniformly welded to each other resulting in, defective welding.
The method for welding with an ultrasonic generator is a method, wherein ultrasonic waves generated from an ultrasonic wave generator are applied to adherends of thermoplastic resin and the adherends are heated at a boundary surface of the adherends and are welded each other by vibration energy of the ultrasonic waves. The above method has advantages in that a defective appearance is not generated due to welding since only the boundary surface of the adherends is welded. Further, the adherends are easily welded to each other since the adherends are not directly heated. However, the above method has a defect in that adherends made of soft materials such as polybutadiene, low density polyethylene and styrene-butadiene elastomer are not heated and are not molten by ultrasonic waves. Thus, the method cannot be applied to adherends made of the soft materials. Also, in the case of medical containers such as bags for injection of liquids and blood bags which are particularly strictly regulated as to the particulates attached to the surface of the containers, shape, material, properties and the like and which are required for high safety and are produced with an ultrasonic wave generator, particulates are sometimes generated on the surface of the containers by the vibration of generated ultrasonic waves requiring a great deal of labor to remove the generated particulates. Still further, when the adherends are welded, the molten adherends are flown out and the burrs are sometimes formed.
It is generally known that many vinylic polymer materials such as polyolefins are less susceptible to high frequency sealing. The above conventional assembly method for blood bags of polyolefin-based polymer having adequate strength has not yet been produced.
Accordingly, the blood bags have hitherto been produced by extruding a tubular parison of material to be molded and sealing the outer end of the parison; ballooning the parison with relatively low pneumatic pressure; closing the mold chamber of the mold; and sealing the parison to form the flexible collapsible bag, including forming seal lines for the bag, by means of the mold, positioned laterally inwardly from the lateral edges of the parison as disclosed in the specification of U.S. Pat. No. 4,191,231.
As mentioned above, although there have hitherto been known various methods for welding thermoplastic resin, a method which enables to easily, uniformly and steadily weld the thermoplastic resin which cannot be welded by a conventional method with a high frequency welder has not yet been developed.
The present inventor has made studies to solve the above-mentioned problems and to develop a method which can easily, uniformly and steadily weld thermoplastic resins such as polyolefin series resin and acryl-butadiene-styrene resin which cannot be welded with a high frequency welder. As the results of his research, he has eventually found the above method and the present invention has been accomplished.
These and other objects of the present invention will become apparent from the description hereinafter.